Method for testing and locating faults in insulation of an electrical apparatus and utilization of the method

ABSTRACT

A method of testing insulation and pinpointing faults in a winding of electrical apparatus wherein by adjusting a test voltage, a change in the potential difference between the winding and an electrically conductive part is brought about. The purpose of this is to effect a striking and extinction of an electrical discharge at a point between the winding and electrical conductive part which exhibits a weak insulating characteristic which is detected by means of a discharge-indicating device.

United States Patent [1 1 Tan [451 Aug. 14, 1973 METHOD FOR TESTING ANDLOCATING FAULTS IN INSULATION OF AN ELECTRICAL APPARATUS AND UTILIZATIONOF THE METHOD [75] Inventor: Tjhing Thian Tan, Baden,

Switzerland [73] Assignee: Aktiengesellschait Brown, Boveri &

Cie, Baden, Switzerland [22] Filed: June 1, 1971 [21] Appl. No.: 148,535

30 Foreign Application Priority am June 4, 1970 Switzerland 8364/70 52U3. Cl. 324/52, 324/54 [51]' Int. Cl. G0lr 31/10, GOlr 31/12 [58] Fieldof Search 324/51, 52,54

[56] References Cited OTHER PUBLICATIONS Ganger et al., IonizationMeasurements on Transformers Ofiprint from the Brown Boveri Review 1967,No. 7. PP- -l5.

Primary Examiner-Gerard R. Strecker Attorney-Pierce, Scheffler & Parker[5 7 ABSTRACT A method of testing insulation and pinpointing faults in awinding of electrical apparatus wherein by adjusting a test voltage, achange in the potential difference between the winding and anelectrically conductive part is brought about. The purpose of this is toeffect a striking and extinction of an electrical discharge at a pointbetween the winding and electrical conductive part which exhibits a weakinsulating characteristic which is detected by means of adischarge-indicating device.

12 Claims, 12 Drawing Figures p p U +A U komp.

sl'rsaloav PAIENIEM: 14 ms SHEEI 3 [If 4 VARIABLE TEST VOLTAGE STABILIZE R PULSE C DISCHARGE INDICATING 0 K INSTRUMENT AUX. VOLTAGBANDSPREAD STABILIZER g ADJ- SHAPER JEZ] 4-TERMINAL DEVICE DIGITALDISPLAYS ELECTRONIC DIVIDER INTERVAL SELECTOR PREPROGRAMMED AUTOMATICPINPOINTING ROUTINE common. PANEL T0 SELECT TEST VOLTAGE) SWITC H ANDADJUST 6 DIGITAL} DISPLAY Fig. 9

Fig.40

mtmiomma 3.753.087

SHEEI t (If 4 VARIABLE TEST VOLTAGE TEST VOLTAGE STABlLIZER wow) D N1 2-0F'(C F cos INDICATOR AUX. VOLT. 4-TERMINAL PULSE UKOMP DEVICE SHAPER ygg- B'(A') K' 01 0| SELECTOR 5 1 ?7 o 8 CI(BI) C| 7L SW5? INTERVALDISCHARGE SELECTOR INDICATOR I I J PREPROGRAMMED AUTOMATIC [0:63 I]PINPOINTING ROUTINE -D|G|TAL DISPLAY fi 6V 1,000 2 E 45 fmmnm DISPLAYLL21! 43 DIGITAL 14 DISPLAYS CONTROL PANEL T0 SELECT AND ADJUST 6 Fig.12

METHOD FOR TESTING AND LOCATING FAULTS IN INSULATION OF AN ELECTRICALAPPARATUS AND UTILIZATION OF THE METHOD The present invention relates toa method of testing insulation and pinpointing faults in an electricalapparatus incorporating at least one winding, this in fact between anelectrically conductive component and a winding or coil which is to betested, the local voltage distribution along the winding as well asalong the electrically conductive component, or one of the two saidlocal voltage distributions, and the difference between them, beingknown in respect of the voltages employed in testing and pinpointing(troubleshooting).

Already, several methods of testing insulation and pinpointing weakpoints in the dielectric where electrical discharges, for example coronadischarges or partial discharges, can occur, this in electricalapparatus containing windings and in particular transformers, are known.

One known method is based upon an acoustic principle, measurements ofthe transit time of theailtrasonic waves produced by electricaldischarges, being made. This method has the drawback that it cannot beapplied in practice to discharge locations in the winding block, betweenthe winding and the ion core or any yoke which is provided, in drytransformers or other apparatus containing windings, with or without anon-liquid filled housing, e.g. having gas or araldite insulation.Furthermore, the ultrasonic noise generated by the transformer generallyswamps the ultrasonic noise of the electrical discharge and thus makesit possible to detect the latter unless extremely elaboratesignalprocessing equipment is used to select the desired ultrasonicsignal. In order to pinpoint a single partial discharge location,several measurements or measurement channels are needed, fortriangulation purposes.

Another known method is based upon bombardment or irradiation withhigh-energy rays, e.g. x-rays, a reduction in the starting voltage andan increase in the intensity of the electrical discharge when the faultlocation is bombarded, being encountered. This method has the drawbackthat it cannot be employed to detect faults located between the windingand the core, within the winding block itself and in situations wherethe housing of the transformer or the apparatus containing the windingsis metal and too thick. Any effective pinpointing is rendered virtuallyimpossible in these circumstances not only by the scattering of theadditional radiation when it encounters metallic components, but alsobecauseof the associated secondary radiation.

Another known method is based upon the measurement of the difference intransit time between the pulses produced by the electrical discharges,e.g. partial discharge, as a consequence of the capacitive voltagecomponents and the travelling wave components, or between the pulsesofthe travelling wave components at the two ends of the winding. Thismethod cannot be employed in practice in situations where the windingsare extensively interleaved, in windings provided with other deliberatecapacitive control arrangements, e.g., with control plates, or where thewinding is highly non-uniform and the measuring system does not presentadequately low reflectivity so that fictitious partial dischargelocation can be simulated.

Another known test method is based upon measurements of the attenuationratio between the travelling wave currents or voltages appearing at thetwo ends of the winding, this either in the real-time range (widebandmethod), or in the frequency range (selective measurement frequency).This method has the drawback that it is virtuallyout of the question touse it in the case of extensively interleaved windings or windings withsome other deliberate capacitive control, e.g. control plates. In thecase of partial discharge locations occurring near the ends of thewindings, this method likewise falls down if the procedurein accordancewith Harold and Sletton is adopted, using the frequency 9: VEarthcapacitance/Series capacitance and in the case of single-layer windings,it is virtually out of the question to employ this method.

Again, when using direct measurement of amplitude, the formulaeavailable are highly approximative ones, and this constitutes anobstacle to satisfactory pinpointmg.

Yet another known method is based upon the measurement of the damped,relatively low-frequency natural oscillations of the overall system(including the measuring arrangement), this either by determination ofthe logarithmic detriment, by setting to zero using a bridgearrangement, or by direct measurement of amplitude. This procedure hasthe drawback that it is sensitive to the measuring arrangement itselfsince the signiticant natural oscillation is not determined exclusivelyby the test subject. Furthermore, measurement involves the use of highlyapproximative formulae which neglect the continuously distributedelements, since otherwise an unacceptable degree of outlay would result,replacing these instead by lumped elements about whose quantitiveequivalents there is some doubt. In the case of rapid successions ofpartial discharge pulses, the measurement is made substantially moredifficult still. The bridge method, considering for example transformertesting using an induced voltage, cannot be applied in practice. Thenecessity, where the bridge method is concerned, of using a high-voltagecapacitor variable in a ratio of about 1 to 21 and down to IOpF, makesthe method difficult to put into practice and is therefore onlyapplicable to low voltages.

The object of the present invention is to create a method which does notexhibit the aforesaid drawbacks of the known methods, is simple andreliable in principle, and fundamentally makes it possible to carry outthe pinpointing operation automatically.

The term electrical discharges shall be understood hereinafter asmeaning for example glow discharges or partial discharges and, in somecircumstances, perforations or flash-overs.

The method of the invention is characterized in that by adjusting a testvoltage, a change in the potential difference between the winding andthe electrically conductive part, and thus the striking and extinctionof an chosen that the local voltage distributions (referred to as testvoltage clearance) assignable to the winding and to the electricallyconductive component, come to be disposed between the previouslydetermined striking and extinguishing limit voltage curves as close aspossible (in accordance with the attainable accuracy measurement and thestability of the auxiliary or test voltage) to the striking orextinguishing limiting voltage curves, depending upon whether thestriking or extinguishing of the electrical discharge is used as theindication of the fault pinpointing; and in that and furthermore, alongthe winding and along the electrically conductive component, by means ofan auxiliary voltage, new local voltage distributions and the differencebetween them (referred to as superimpositon voltage curves andsuperimposition voltage difference curves) are developed as aconsequence of the common influence of auxiliary voltage and testvoltage in such a way that the ratio between superimposition voltagedifference curve and striking or extinguishing limit voltage differencecurve is never constant along the winding, even if auxiliary voltageand/or test voltage are further adjusted, thereupon, by adjustingauxiliary voltage and- /or test voltage in a suitable direction, thesuperimposition voltage difference curve, at the point which correspondsto that position in the winding which is to be pinpointed and whichexhibits weak insulation, being brought to an equivalent value(depending upon whether the striking or extinguishing of the electricaldischarge is to be taken as the indication of the pinpointing of thefault) to the magnitude of the striking limit voltage difference curveor the extinguishing limit voltage difference curve at the same point,but now however corresponding with the striking or extinguishing limitvoltage difference curve at other points, on the one hand, and on theother hand, in accordance with the attainable accuracy of measurementand the stability of auxiliary or test voltage, not having to depart toofar therefrom (this at any rate as far as possible), the attainment ofthis adjusted condition being marked, during the business of adjustingusing the auxiliary voltage and/or test voltage, by the striking orextinguishing of the electrical discharge and thus, from thecharacteristic magnitudes of auxiliary voltage and test voltage appliedto this striking or extinguishing time, from the striking orextinguishing level (of the test voltage) belonging to the striking orextinguishing limit voltage difference curve, and from the number ofturns in the winding tested. that point in the tested winding whichexhibits weak insulation and where the electrical discharge has takenplace, being determined.

It is convenient to induce the test voltage in the winding which is tobe tested, to connect the voltage source for this induced test voltageto a further winding coupled to the winding under test or, for examplein the case of apparatus with only one winding, to connect it directlyacross the winding ends and, in order to pinpoint a fault which hascaused an electrical discharge between the tested winding and anelectrically conductive component, to apply the auxiliary voltagebetween said electrically conductive component and one end of the testedwinding.

It may also be convenient to apply the test voltage across the ends ofthe short-circuited winding or between one end of the open circuitwinding and an electrically conductive component and, in order topinpoint a fault which has produced an electrical discharge between thetested winding and the electrically conductive component to induce theauxiliary voltage in the tested winding, the short-circuiting of thelatter, provided that its ends were not already open-circuited duringthis voltage test, having to be cancelled and the voltage source for theinduced auxiliary voltage being connected to a further winding which iscoupled with the winding under test, or directly across the ends of thesaid tested winding.

Preferentially, in actually pinpointing the fault, especially where thestriking of the electrical discharge is used as the indicator that thefault has been pinpointed, the test voltage adjusted betweenthe-striking and extinguishing voltages of the electrical discharge willbe so chosen that the requisite auxiliary voltage is as small aspossible.

The subject of the invention is furthermore an application of the methodof the invention to insulation testing and fault pinpointing in atransformer, a measurement transducer, a choke coil or a solenoid coil.

In the following, the invention will be explained making reference toseveral examples of application, illustrated in the drawing, and indeedtaking the case of the occurrence of partial discharges between awinding and the housing of a transformer, pinpointing being carried outusing a.c. voltages.

FIG. 1 is the fundamental diagram of an arrangement for carrying outinsulation testing using an induced voltage;

FIG. 2 is a fundamental diagram similar to that of FIG, 1, relating thistime to the pinpointing arrangement;

FIG. 3 is the voltage diagram when employing the pinpointing arrangementof FIG. 2, with the partial discharge striking voltage U, as theindicator;

FIG. 4 is the voltage diagram obtained when using the pinpointingarrangement of FIG. 2, with the partial discharge extinguishing voltageU asthe indicator;

FIG. 5 is a fundamental diagram of the arrangement for carrying outinsulation testing by an applied voltage;

FIG. 6 is a fundamental diagram similar to that of FIG. 5 showing thepinpointing arrangement;

FIG. 7 is the voltage diagram obtained when using the pinpointingarrangement of FIG. 6, with U, as the indicator;

FIG. 8 is the voltage diagram obtained when using the pinpointingarrangement of FIG. 6 with U, as the indicator;

FIG. 9 is a block diagram of an automatic test and pinpointing devicefor implementing the methods described in FIGS. 1 to 8;

FIG. 10 is a fundamental diagram similar to that of FIG. 2, of thepinpointing arrangement with phase regulator;

3,753,087 I t t FIG. 11 is a fundamental diagram similar to that of FIG.6, of the pinpointing arrangement with phase regulator; and

FIG. 12 is a block diagram of an automatic test and pinpointing devicefor carrying out the methods described in relation to FIGS. and 11.

In the ensuing explanations E, applied test voltage across primarywinding (N,

turns); only used when testing with induced voltage.

U, test voltage across the secondary winding being tested (N turns) Whenusing an induced voltage for testing, the relationship U, N /N E,applies.

U, value of test voltage U, at which partial discharge strikes.

U value of test voltage U, at which partial discharge extinguishes.

U auxiliary voltage used to compensate a change in U,.

Let us assume that the point X-X' in the secondary winding is thepartial discharge location corresponding to a number of turns N wherethe local voltage between x and the end of the winding which isconnected to the earthed housing is U, ai /N2) U,, and that we areconcerned here with a partial discharge from a point in the windingtested to a conductive part of the apparatus (the housing), which is atzero potential.

In FIG. 1 the fundamental diagram of an arrangement for insulationtesting using an induced voltage, is illustrated. If we substitute forthe continuous connection BC, an auxiliary voltage of the same frequencyas the test voltage, then we obtain the fundamental diagram of FIG. 2describing the pinpointing arrangement for carrying out testing withreduced voltage.

At commencement (striking) of the partial discharge when Up U, itfollows from FIGS. 1 and 2 since U, (N /N U,, that for the partialdischarge striking voltage at the point X-X', we obtain As the testvoltage U, is increased, it is possible, for example using a known kindof narrow-band parasitic voltage measuring device, quite simply todetermine the striking of the partial discharge and therefore the U,value and, because the pattern of the voltage distribution along thewinding is assumed to be known a priori, the associated striking limitvoltage curve a (FIG. 3) assigned to the secondary winding, is known aswell.

U, is then reduced until the partial discharge extinguishes, from whichU, is obtained and, because the pattern of the voltage distributionalong the winding is known, it is also possible to determine theextinguishing limit voltage curve b (FIG. 3) assigned to the secondarywinding. After the extinguishing of the partial discharge, the testvoltage U, is raised until there is the smallest possible AU, valuevis-a vis U, so that a new voltage curve, the test voltage curve 0 (FIG.3), is obtained which lies between the two previously determinedstriking and extinguishing limit voltage curves a and b and is assignedto the secondary winding. If, then, a co-phasal auxiliary voltage U isapplied across the terminals B and C (FIG. 2), a new resultant voltagecurve, the superimposition voltage curve d (FIG. 3) is obtained, whichis now assigned to the tested secondary winding. Here, at the pointX-X', we have the condition s: am/ 2) ra Ukanw The voltage U,,,,,,, isthen varied until the condition U, U obtains, whereupon the partialdischarge strikes again. From this, it follows that i 0 ana 2) komu s Inthis fashion, the faulty turn at the point X-X' can be determineddirectly from the ratio of the measured voltages U,,,,,,,, and U, andthe total number of turns in the winding. Because, when using thestriking of the electrical discharge as the indicator which marks thepinpointing of the fault, the condition U, U,,,,,,,, U applies at thepoint y-y (FIG. 3, when U, U the auxiliary voltage U,,,,,,,, which isrequired should be as small as possible in order with a probabilitybordering on certainty not to cause any other potential partialdischarge locations, e.g. at 17 (something which in transformers withstepped insulation, is not inconceivable under unfavourablecircumstances), to produce discharges. In transformers with fullinsulation, where the winding has an equal thickness of insulation overits whole length, the possibility of this happening is virtuallycompletely excluded. Nevertheless, in order as far as possible tomaintain the original condition, in pinpointing the fault the testvoltage, depending upon what selection of indicator is made in order toestablish that the fault has been pinpointed, should be adjusted soclose to the striking or extinguishing voltage that the smallestpossible auxiliary voltage is needed to actually effect pinpointing.Self-evidently, it is equally possible, instead of the auxiliary voltageU to adjust the test voltage U, until the condition U, U,,,. isobtained.

If the position of the resultant superimposition voltage curve d ischanged through adjustment of the test voltage U,, it is advantageous topreviously select a relatively low auxiliary voltage U,,,,,,,.

In the following, the same method of pinpointing, this time howeverusing the extinguishing of the partial discharge of the indicator, willbe explained making reference to FIGS. 1, 2 and 4. In accordance withFIG. 2, a switch 18 is provided for the purpose, this in order tosubtract the test and auxiliary voltages from one another instead ofadding them. I

U, and U, are determined in the same way as in the preceding example sothat the striking limit voltage curve a and extinguishing limit voltagecurve b assignable to the winding under test, are known.

When U, U,,, the partial discharge extinguishes, i.e. the partialdischarge extinguishing voltage at the point X-X' is var (NM/N.) v.

Subsequently, U, is briefly raised again to somewhat above U, so thatthe partial discharge strikes again. After this has happened, the testvoltage is lowered again until there is the smallest possible AU, valuevisa-vis U,., so that a new voltage curve, the test voltage curve a(FIG. 4), which is located between the two previously determinedstriking and extinguishing limit voltage curves at and b and isassignable to the secondary winding, is obtained, this representing theadjusted condition. If, then, an antiphase auxiliary voltage U isapplied across the terminals B and C (FIG. 2), a new resultant voltagecurve (the superimposition voltage curve d (FIG. 4), is obtained, whichis now assigned to the secondary winding under test. In this context, wehave the condition U: M/ 2) e A p) komn at the point X-X. The auxiliaryvoltage U,,,',,,, is then altered until U, U whereupon the partialdischarge extinguishes again, from this it follows that M/ 2) ta konw u/2) or 2..r/ 2) UROMP/AUP In this way too it is possible to calculate theposition X-X' of the faulty turn, relatively simply. Selfevidently, itis possible here, too, to regulate the test voltage U, instead of theauxiliary voltage U,,,,,,,, until the condition U, U applies, or toregulate both said voltages.

In FIG. 5, by way of another example the fundamental diagram of anarrangement for insulation testing by means of an applied voltage, isillustrated. If an auxiliary voltage of the same frequency as the testvoltage is applied across the pair of terminals AB, then the fundamentaldiagram illustrated in FIG. 6, of the pinpointing arrangement used fortesting with an applied voltage, is obtained, i.e. in which the testvoltage is applied between the secondary winding of the test, and theprimary winding of the housing. Here, as FIG. 2 shows, once again aswitch 18 and a voltage regulator 19 are provided. In FIG. 6, as in FIG.2, all the bracketed references relate to the use of U, as theindicator.

As the best voltage U, is raised, here again, as already described, thevoltage U, can be determined by means of a partial discharge parasiticvoltage measuring instrument, this being the voltage at which a partialdischarge strikes; in this way striking limit voltage curve a (FIG. 7)is obtained which can be assigned to the secondary winding.

Thereafter, U, is reduced again until the partial dischargeextinguishes, thus determining U, and therefore the extinguishing limitvoltage curve b (FIG. 7) belonging to the secondary winding.

At the partial discharge location X-X', as FIG. 7 shows, we have thecondition After the partial discharge has extinguished, the test voltageU, is raised again to give the smallest possible AU, value vis-a-vis U,,so that a new voltage curve, the test voltage curve c" (FIG. 7) locatedbetween the two previously determined striking and extinguishing limitvoltage curves 0" and b", is obtained, this new curve being assignableto the secondary winding. If, then, between the terminals A and B (FIG.6), a co-phasal auxiliary voltage U,,,,,,, is induced, a new resultantvoltage curve, the superimposition voltage curve d (FIG. 7), isobtained, which is now assignable to the tested secondary winding. Here,at the position X-X, the condition applies. The auxiliary voltageU,,,,,,, is then altered until the condition U, U, is obtained,whereupon the partial discharge strikes again at the position X-X. Fromthis it follows that With the aid of this formula, after thedetermination of AU, and U,,,,,,,, the faulty position in the windingcan be calculated directly from the total number of turns N, in thetested winding. Self-evidently, it is equally possible instead of theauxiliary voltage U,,,,,,, to so adjust the test voltage U, that U U,obtains, or to adjust both voltages. For reasons similar to thoseapplying in the case of testing and pinpointing with an induced testvoltage, as described in FIG. 2, likewise when testing and pinpointingusing an applied test voltage AU, should preferably be chosen as smallas possible so that the requisite auxiliary voltage U,,,,,,, is kept toa minimum as well.

If the extinguishing of the partial discharge is used as the indicator,then first of all U, and U, and therefore the striking and extinguishinglimit voltage curves 0" and b (FIG. 8) are determined, in the samemanner as in the foregoing example. Then, U, is again briefly raised toabove U, so that the partial discharge strikes again. After thishappens, the test voltage U, is reduced again to give the smallestpossible AU, value vis-a-vis U,, so that a new voltage curve, the testvoltage curve c (FIG. 8) which is located between the two previouslydetermined striking and extinguishing limit voltage curves a" and b", isobtained, which new curve is assignable to the secondary winding andrepresents the adjusted condition. If, then an antiphase auxiliaryvoltage U,,,,,,, is induced across the terminals A and B (FIG. 6), a newresultant voltage curve, the superimposition voltage curve 41" (FIG. 8),is obtained, which is then assignable afresh to the tested winding. Theauxiliary voltage U,,,,,,,, is then so modified that U, U, obtains,whereupon the partial discharge extinguishes again. Here, we have Withthe help of this formula, it is possible, as in the foregoing exampleand after determination of AU, and U,,,,,,,,, to calculate the faultypoint in the winding from the total number of turns N, of the testedwinding. Here again, it is of course possible instead of the auxiliaryvoltage U,,,,,,, to so regulate the test voltage U, that U, U, or forthat matter to regulate both voltages.

In this method using U, as the indicator, it is not possible, eitherwhen testing with an induced voltage or with an applied voltage, for anyother partial discharge location to occur because during pinpointing nopart of the winding is subjected to a voltage which is any higher thanobtained in the condition under which the partial discharge struck forthe first time.

The electrical discharge may be located not, as in the precedingexamples, in the space between the tested winding and the housing, butquite generally in an insulation space between the tested winding andsome electrically conductive component, for example in the space betweenthe tested winding and another winding, a part of the magnetic circuitof the apparatus, the adjacent effective earth (in the absence of anyelectrically conductive housing), or also for that matter componentsinstalled inside the apparatus as for example screening arrangements,supply leads, switch components, control elements and so on. The faultpinpointing procedure in accordance with the invention can in principlebe applied to all these cases. If, in addition to the tested winding,the electrically conductive component (for example another winding)which bounds the insulation space in which the discharge between saidelectrically conductive component and the tested winding has takenplace, also has a known voltage distribution in respect of the test andauxiliary voltages employed in testing and pinpointing, then thedifference between the voltage distributions, i.e. the curverepresenting the difference between the striking or extinguishing limitvoltages, the superimposition voltages, etc (of tested winding andelectrically conductive component), is of critical importance in thepinpointing operation.

The test voltage and the auxiliary voltage need not necessarily have asinusoidal waveform. It is equally possible to use pulse voltages whichhave linear or nonlinear but no local voltage distributions in the coil,or again repetition voltage processes of arbitrary form, e.g.half-waves, likewise with known local voltage distributions. equally, asfar as test and auxiliary voltages are concerned, it is not absolutelyessential to use voltages of the same type. Moreover, one of thevoltages could be a direct one. For example, when carrying out faultpinpointing with an induced voltage (FIG. 2) as the test voltage, theauxiliary voltage can be direct voltage, whilst when testing with anapplied voltage (FIG. 6), a direct test voltage is quite conceivable. Inthe case of an ac. voltage, test voltage and auxiliary voltage need notnecessarily have the same frequency.

When using the above-described different kinds of voltages, theestablishment of fault pinpointing, can in principle be arrived at notonly by amplitude adjustment of auxiliary and/or test voltages, but,given a suitable voltage form, equally by shifting the relative phasepositions of auxiliary and test voltages, or, if one of them is a pulsevoltage, by shifting the synchronisation trigger point of the pulsevoltage or voltages. Again, a combination of amplitude control andtime-shift in auxiliary and test voltages, is possible.

If the apparatus has only one winding, then for example the housing canbe regarded as a second winding with zero turns and the induced voltage"then means that voltage which is applied directly across the windingterminals. The applied voltage will then be that voltage which isapplied between the single winding and the housing.

Where there are several windings, for testing and possible pinpointing,it is possible to deal with the windings in successive pairs. Thewindings which are in each case not involved by the particular testoperation, must thenbe open-circuit and connected to the housing.

In FIG. 9, the block diagram of an automatic test and pinpointing devicefor implementing the aforedescribed examples of the method of theinvention, is illustrated. Here l variable test voltage source 2 testvoltage stabiliser, temporarily (during pin pointing) operative andcontrolled by unit 6.

3 switch, controlled by unit 6, for switching the auxiliary voltage fromAddition to Suhtrao tion" when a change in indicator from U. to 1.1., ismade. I

4 auxiliary voltage stabiliser, which can be disconnected by unit 6.

5 bandspread fine adjustment of the auxiliary voltage.

6 prcprogrammed automatic pinpointing routine,

with store.

7 selective or matching preliminary four-terminal device associated withdischarge indicating instrument.

8 discharge indicating instrument, for example partial dischargeparasitic voltage measuring instrument.

electronic pulse-shaper unit with U U, discriminator action, producingcontrol pulses for processing in routiner 6.

10 electronic divider for calculating (AU /U or kmm p) 1 l digitaldisplay of the pinpoint N /N 12 digital display or/and recorder for U 13digital display or/and recorder for U 14 control panel for selection andadjustment of the automatic test and pinpointing routine.

15 arrangement for selecting a specific interval in the voltage-timecharacteristic.

The terminals A, B K of the test subject are intended for connection tothe similarly designated terminals A',B K of the automatic test andpinpointing device. Between K'C and unit 7 (FIG. 9), it may beadvantageous, for purposes of detection of the discharge signal, toconnected a device which selects a specific interval of variable length.in the voltage-time characteristic of the tapped off discharge signal,this without placing any attenuative load at the equipment side of theoutput KC for the discharge signal, on the discharge indicatinginstrument at unit 7. The heart of the automatic device is the automaticroutiner (unit 6) which is responsible for controlling the fouroperating routines, in fact for carrying out pinpointing with an appliedor induced test voltage, using U, or U, as the indication. Through theagency of the control panel 14, the unit 6 is set to one of these.routines. The pulses produced by the pulse-shaper unit 9 from the outputsignal furnished by the discharge indicating instrument, for example apartial discharge parasitic voltage measuring instrument, are employedin the unit 6 to control the pinpointing routine and to determine thetimes (instants) of the significant voltage readings. The afore saidpulses and voltage readings are stored in the store of unit 6 forensuing processing and output.

Self-evidently, it is equally possible to carry out the aforesaid testand pinpointing operations by means of phase control, if the test andauxiliary voltages required for the pinpointing operation are ac.voltages of like frequency.

Using a procedure similar to that employed when testing with inducedvoltage and carrying out pinpointing by amplitude adjustment of theauxiliary and/or test voltages. as in FIGS. 1 and 2, in testing with aninduced voltage and carrying out pinpointing using phase adjustrncnts,in accordance with FIG. 10, first of all the striking or extinguishingvoltage U (an abbreviated convention for U or 1.1,), is determined. Thetime function of the discharge striking or extinguishing voltage, at thepoint X-X where the insulation fault is occurring, then reads ttem' @995m (Nani/Ne) Um 2 (2308 ml The test voltage is then mcdiiicdtc U a U *1AU, where A, in pinpointing arrangements using phase adjustment, has apositive or negative sign. AU, or AU, 0, depending upon whether thestriking or extinguishing of the electrical discharge is to be used asthe indicator of the fact that the fault has been pinpointed, and, too,]A U,,| will advantageously be selected as small as possible just as inthe aforedescribed test and pinpointing limits (N g/N AU,,/(2U AU,) andpinpointing arrangements. Across the terminals B and (N /N 1. All faultlocations with N /N values C P q y an auxiliary Voltage komp betweenVAU,,/(2U,+AU,, and 1, can thus in princi- 2-cos (on IS applied, where4: can be adjusted using ple be pinpointed if d) is varied from 1r/2 to11'. Considera phase regulator (phase-shift device) 21. In this con- 10ing a fault location for which (N /N \/AU,,/(2U

indicating the text, the adjustment of (1) will preferably commence AU,)holds, the phase-shift must be carried out with from dr- 11' or (b 11/2,if t e t g 0r e t guis ing, a reduced U value, e.g. half the value. Withthe respectively, of the electrical discharge is to be used asconsequent expansion of the bottom pinpointing limit, the indicator ofthe fact that the fault has been pinhowever, the must now be takenaccount of in expomted (i.e. at AU O or AU 0). We then obtain 15pression (1). Taking into account the and in exfor the superimposedvoltage at the point X-X' (FIG. pression (I), with U AU, the extendedpinpointv I u ing range moves to a position between the pinpointing UV2Tcos (wt+8) 2 (U +A,,) Vi cos wt+U Vi cos (wt+) komn 2 If qb isadjusted so that A becomes I 7 limits (N /N Ay /(2U AU,,) and (N /N W2..z'/ 2) m) m) m): a 2,.r/ 2) p hether or not m a given pinpointingcase, the or the sign is to be taken (2 (Nz-z/Nz) 2 (UM cos account ofin expression l will be indicated by a second measurement using asomewhat smaller U (quadratic equation in terms 0f 2,.r/ 2) then thestrikvalue. With N /N as an invariable, in accordance ing orextinguishing of the electrical discharge provides with expression (1) asmaller U will yield a more the indication that the adjusted conditioncorrespondnegative or less negative cos 4: value in pinpointing, deingto the pinpointing of the fault, has been reached. di upon h h h or i 1iT By solving the above quadratic equation for N2.1/N2 tom pinpointinglimit in trying to locate the fault posi- N2 AUp 2 Uf(e)+ AUp the turnat the point X-X. which contains the faulty tion y Shifting the Phasefrom t can be ght insulation, can fundamentally be determined from Uarbitrarily Close to u/ 2) the P ase-Shift on N, and the values of AU, Ud I, hi h are tech- 40 each occasion being continued with a reduced Unically not mutually exclusive. If, therefore, the expres- Valuesion (1)for Nm/N is to be furthet technically inter I In the event that thestriking of the discharge has preted, then, because of the conditions inaccordance been used as the indicator for the pinpointing, at

' with Ai /N 0 and the discriminate of the quadratic AU O s cos 1), inexpression (1) only the equnon these f Imposed by i' negative sign alies since N /N cannot be negative. erations, we must have either AU,0.1.8.. extinguish- I UM" V=H H my; gives afjpwinpoinlting ing, of thedischarge as the pinpointing lndicator, range 'b fi (N ,/N l and ae/ 2KU,7(2U AU, and in pinpointing only negative cos (b values can occur.With a smaller U,,,,,,,, value,

U A U l AU, I the limiting values cos d: l and +1 yield an expandedpinpointing range between the limits u/M) -AU,/(2U +AU,) and (N /N l.The bottom pinpointing limit can here again be brought arbitrarily closeto Ngg/NQ 0 by carrying out the phase-shift from 1r to 21r (or 0) ineach case with a reduced (e.g. halved) U value. For similar reasons tothose applying to testing with an induced voltage and pinpointing withamplitude control of the thus the extinguishing of the discharge as thepinpointing indicator the preferred starting point of the 4: shift willbe 17/2; a phase-shift of 1r/2 to 11'; or

AU, O i.e. the striking of the discharge is used as the indicator inpinpointing;

- 1 5 cos 1 in association with AU,I 0 the starting point of the dashiftwill preferably be 1r; a phaseshift from 1r to 211' or 0.

In the first case, using the extinguishing of the discharge as thepinpginting indicator, with an auxiliary In FIG. 11, by way of anexample a fundamental diavoltage \lU AU, (2U, AU; selected by the gramsimilar to that if FIG. 5 has been shown. relating amplitude selector20, it follows from the condition 0 65 this time to a pinpointarrangement employing phase (N2..r/Ng) 1, that the expression (1) isonly valid adjustment in the context of testing with an applied if thesign is the limiting values of cos (b l and voltage.

should be selected as small as possible so that the requisite auxiliaryvoltage U can also be made small.

auxiliary and/or test voltages (FIG. 2), here again |AU,|

By a similar procedure to that employed with testing by an appliedvoltage and pinpointing with amplitude control of the auxiliary and/ortest voltages (FIG. here again, first of all the striking orextinguishing voltage U is determined. The time function of thedischarge striking or extinguishing voltage at the point X-X' where theinsulation fault is located, runs as follows, considering testing withan applied voltage U Vi. cos wt U, Vi cos wt.

The test voltage (FIG. 1 1) applied bet'ween the winding and point C, isthen altered to U, U A U,, where A here, in pinpointing arrangementsusing phase adjustment, has a positive or negative sign, i.e. AU, 0 or 0depending upon whet-her the striking or extinguishing of the electricaldischarge is to be used as the indicator of the fact that the fault hasbeen pinpointed. Also, iAU, l as in the previously described test andpinpointing examples, should be chosen as small as possible. In thewinding, subsequently, an auxiliary voltage U \/f-cos(wz is inducedbetween the points A and B (FIG. 11), d) being variable by means of aphase-shift device 21. The adjustment of 4) will preferably commencefrom the starting point d: 1r/2 or (I) 'n', depending upon whether theextinguishing or striking of the electrical discharge is to be used asthe indicator of the fact that the fault has been pinpointed. We thenobtain from the superimposed voltage at the point XX' (FIG. 11):

AU, 0 the indication of pin-pointing is the extinguishing of thedischarge,

'rr/2 being the preferable starting point for the shift in d); aphase-shift from 1r/2 to 1r; or

A U, O pinpointing indication in the form of striking of the discharge,-1 s cos 4 1 11' being the preferential starting point of the d shift;phase-shift from 1r 277 or O. In the first case, where the extinguishingof the discharge is taken as the indication that pinpointing has beenachieved, with an induced auxiliary voltage of U V AU,,(2U +AU,) betweenthe points A and B of the winding (FIG. 1 1), it followsfrom theexpression 0 (N /N s I that exclusively the sign is valid in expression(2). Taking the limiting values cos d l and komp V2 cos (cot-lb) n i' Hcos komn (quadratic equation in terms of (N /N then the striking orextinguishing of the electrical discharge constitutes the indicationthat the adjusted condition corresponding to the pinpointing of thefault, has been reached.

Solving the quadratic equation for 2,.r/ g we'obtain N Ukomp (U -l-AU,,) cos 4: becomes a pinpointing range'with the range limits (N /NVAUp/(ZU, AU,) and (NM/N :1 is obtained. In the case of fault locationsat which (Nu/N vKu,,/ 2u, AU,,) applies, the phase-shift must becontinued with a reduced AU, value and a correspondingly reduced value UVAU, (2 U, AU,)

The lowering of the bottom pinpointing limit can also be achieved bymaking U larger for the same AU,, although this is not generallyadvisable. For example, taking U 2 U +A U,,, although a widerpinpointing range would be obtained, with pinpointing limits of (Na /N3)Ng lNg 2 kumn and thus the curve exhibiting the weak insulation atcorresponding to coP-l both and signs would point X-X', canfundamentally be determined from! U N and the values of AU,, U and Awhich are technically not mutually exclusive.

Similarly to the preceding example in accordance with FIG. 10, here,too, because of the conditions in accordance with which (N /N 0 and thediscriminant of the quadratic equation 2 0, it follows that theexpression (2) to be capable of application, either have to be takenaccount of (2). Which sign is to be used in a given pinpointing casewould then have to be clarified by a second measurement using a somewhatsmaller U value. With Nut/N invariable, then, in accordance withexpression (2), taking a smaller U value in the pinpointing operation amore negative or less negative case would be obtaineddepending uponwhether the or sign applied. Again, with the phaseshift method, two higha U value in the neighbourhood of the starting point could cause aomeother fault location to produce an electrical dischargw, something whichis of course undesirable.

In the event that the striking of the discharge has been chosen as thepinpointing indicator (i.e. U, 0,l cos Q 1), in the expression (2) it isexclusively the sign which applies .because N /N cannot be negative.

With an induced auxiliary voltage of U \/A U ,,(2 U ,-+A U between thepoints A and B of the winding (FIG. 11), because of the condition (N.1/N 1 a pinpointing range, with limits of (N /N vAU,,/(2U+AU,) and, N/N l is obtained and exclusively positive cos d) values can occur inpinpointing. In the case of faultlocations at which N ,/N \)AU,/(2U,+AU; applies, the phaseshift must be continued using a reduced A U, valueand correspondingly reduced value of U The reduction in the bottompinpointing limit can also be achieved by selecting a larger U value forthe same AU value, although this is not generally advisable. Taking forexample U,,,,,,,,,=2U,+AU,,, although a wider pinpointing range withpinpointing limits of N /N --AU,,/(2U,+AU,,) and N,,,/N, 1 correspondingto cos d: ==+1, and cos d: =*l, would be obtained, if U is too largethere is an increased chance that with the phaseshift operation someother fault location will be caused to produce an electrical discharge,and this is of course undesirable. In the examples of pinpointingarrangements shown in FIGS. and 11, the phaseshift devices arranged inthe auxiliary voltage circuit. However, modifications of thearrangements are entirely conceivable, in which the phaseshift device iscontained in the test voltage circuit or in both test voltage andauxiliary voltage circuits.

In the pinpointing arrangement examples of FIGS. 6 and 11, the inducedauxiliary voltage is applied directly across the winding under test,which has N turns. Selfevidently, it is equally possible to apply theinduced auxiliary voltage after the switch 18 (FIG. 6) or after thephaseshift device 21 (FIG. 11), across the second winding if such isavailable (for example the winding with N, turns in FIGS. 6 to 11).

If the electrical discharge take place in the insulation space betweentwo windings, in determining the point which is producing the electricaldischarge the numbers of turns in both windings must be taken intoaccount.

In FIG. 12, the block diagram of an example of an automatic pinpointingdevice for carrying out the methods explained in relation to FIGS. 10and 11, is illustrated. The references here have the followingsignificances:

l variable test voltage source with automatic reducing system controlledby unit 6' (e.g. halving system), for U,,.

2 test voltage stabiliser; only temporarily operative (duringpinpointing), and controlled by unit 6'.

3 U amplitude selector (transformer), with automatic reducing system forU controlled by unit 6.

4' I continuous phaseshift device for the auxiliary I voltage. 5 Iauxiliary voltage stabiliser. controlled by unit 6'. 6 preprogrammedautomatic pinpointing routiner with storer.

7' selective or matching preliminary four-terminal device for thedischarge indicating instrument.

8 discharge indicating instrument, for example partial dischargeparasitic voltage measuring instrument.

electronic pulse-shaper unit with U Urdiscriminating action, suppliescontrol pulses for processing in routine as 6'.

l0= cos d: indication with sign indicator 1 l'= digital display of thepinpointing position 12= display and/or recorder for U,

I3= digital display and/or recorder for U l4= digital display or/andrecorder for I AU,,| with sign indicator.

I5= control panel for selecting and adjusting the automatic testing andpinpointing.

l6= Arrangement for selecting a specific interval in the voltage-timefunction.

I claim:

1. A method of testing an insulated winding incorporated in anelectrical apparatus and pinpointing any fault which may exist in theinsulation between any point along the winding and an electricallyconductive component of the apparatus, the pattern of the voltagedistribution along the winding being known, which comprises the stepsof:

applying a progressively increasing test voltage across. the ends ofsaid winding until a partial discharge strikes between the winding andthe electrically conductive component of the apparatus,

decreasing the test voltage until the partial discharge extinguishes,

again increasing the test voltage to a level just under that at which apartial discharge would again be struck,

adding to said test voltage an auxiliary voltage applied between one endof said winding and the electrically conductive component and which isvaried until said partial discharge re-strikes, and

determining the location of the faulty turn in said winding where thepartial discharge struck from the ratio of the test and auxiliaryvoltages and the total number of turns in the winding.

2. A method of testing an insulated winding as defined in claim 1wherein said test and auxiliary voltages are of the alternating currenttype and have the same phase and frequency.

3. A method of testing an insulated winding as defined in claim 1wherein said test and auxiliary voltages are of the alternating type andhave the same frequency and which includes the step of also varying thephase of said auxiliary voltage relative to that of said test voltage.

4. A method of testing an insulated winding incorporated in anelectrical apparatus and pinpointing any fault which may exist in theinsulation between any point along the winding and an electricallyconductive component of the apparatus, the pattern of the voltagedistribution along the winding being known, which comprises the stepsof: I

applying a progressively increasing test voltage across the ends of saidwinding until a partial discharge strikes between the winding and theelectrically conductive component of the apparatus, decreasing the testvoltage until the partial discharge cxtinguishes, again increasing thetest voltage to a level just above that where the partial dischargewould first strike,

subtracting from said test voltage an auxiliary voltage applied betweenone end of said winding and the electrical conductive component andwhich is varied until said partial discharge extinguishes again, and

determining the location of the faulty turn in said winding where thepartial struck from the ratio of the test the auxiliary voltages and thetotal number of turns in the winding. 5. A method of testing aninsulated winding as defined in claim 4 wherein said test and auxiliaryvoltages are of the alternating current type having the same frequencybut displaced in phase by 180.

6. A method of testing an insulated winding as defined in claim 4wherein said test and auxiliary voltages are of the alternating currenttype and have the same frequency and which includes the step of alsovarying the phase of said auxiliary voltage relative to that of saidtest voltage.

7. A method of testing an insulated winding incorporated in anelectrical apparatus and pinpointing any fault which may exist in theinsulation between any point along the winding and an electricallyconductive component of the apparatus, the pattern of the voltagedistribution along the winding being known, which comprises the stepsof:

applying a progressively increasing test voltage between one end of saidwinding and the electrically conductive component until a partialdischarge strikes between the winding and the electrically conductivecomponent of the apparatus, decreasing the test voltage until thepartial discharge extinguishes, again increasing the test voltage to alevel just under that at which a partial discharge would again bestruck,

adding to said test voltage an auxiliary voltage applied across the endsof said winding and which is varied until said partialdischargere-strikes, and

determining the location of the faulty turn in said winding where thepartial discharge struck from the ratio of the test and auxiliaryvoltages and the total number of turns in the winding.

8. A method of testing an insulated winding as de fined in claim 7wherein said test and auxiliary voltages are of the alternating currenttype and have the same phase and frequency.

9. A method of testing an insulated winding as defined in claim 7wherein said test and auxiliary voltages are of the alternating currenttype and have the same frequency and which includes the step of alsovarying the phase of said auxiliary voltage relative to that of saidtest voltage.

10. A method of testing an insulated winding incorporated in anelectrical apparatus and pinpointing any fault which may exist in theinsulation between any point along the winding and an electricallyconductive component of the apparatus, the pattern of the voltagedistribution along the winding being known, which comprises the stepsof:

applying a progressively increasing test voltage between one end of saidwinding and the electrically conductive component until a partialdischarge strikes between the winding and the electrically conductivecomponent of the: apparatus,

decreasing the test voltage until the partial discharge extinguishes,again increasing the test voltage to a level just above that where thepartial discharge would first strike,

subtracting from said test voltage an auxiliary voltage applied acrossthe ends of said winding and which is varied until said partialdischarge extinguishes again, and

determining the location of the faulty turn in said winding where thepartial discharge struck from the ratio of the test and auxiliaryvoltages and the total number of turns in the winding.

11. A method of testing an insulated winding as defined in claim 10wherein said test and auxiliary voltages are of the alternating currenttype having the same frequency but displaced in phase by 12. A method oftesting an insulated winding as defined in claim 10 wherein said testand auxiliary voltages are of the alternating current type and have thesame frequency and which includes the step of also varying the phase ofsaid auxiliary voltage relative to that of said test voltage.

qg gg UNITED STATES PATENT OFFICE CERTIFKEATE ()F CORRECTION Patent No.3,753,087 Dated August 1, 973

Invent r-fled Tjhing Thrian Ten It is certified that; error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

Claim 1, line 1 9, after "component" insert:

- in ord'erto effect a slight shift in the voltage distribution curveClaim L, line 18, after "component" insert:

in order to effect a slight shift in the voltage distribution curveClaim line 22, after "partial" insert:

discharge Claim 7, line 19, after "winding' insert:

- in order to effect a slight shift in the voltage distribution curveClaim 10, line 18, "after "winding" insert:

- in order to effect a slight shift in the voltage distribution curveSigned and sealed this 20th day of November 1973.

(SEAL) A-ttest:

E D WARD M.FLETCHER,JR. RENE D. IEGTMEYER Attesting Officer ActingCommissioner of Patents 1

1. A method of testing an insulated winding incorporated in an electrical apparatus and pinpointing any fault which may exist in the insulation between any point along the winding and an electrically conductive component of the apparatus, the pattern of the voltage distribution along the winding being known, which comprises the steps of: applying a progressively increasing test voltage across the ends of said winding until a partial discharge strikes between the winding and the electrically conductive component of the apparatus, decreasing the test voltage until the partial discharge extinguishes, again increasing the test voltage to a level just under that at which a partial discharge would again be struck, adding to said test voltage an auxiliary voltage applied between one end of said winding and the electrically conductive component and which is varied until said partial discharge restrikes, and determining the location of the faulty turn in said winding where the partial discharge struck from the ratio of the test and auxiliary voltages and the total number of turns in the winding.
 2. A method of testing an insulated winding as defined in claim 1 wherein said test and auxiliary voltages are of the alternating current type and have the same phase and frequency.
 3. A method of testing an insulated winding as defined in claim 1 wherein said test and auxiliary voltages are of the alternating type and have the same frequency and which includes the step of also varying the phase of said auxiliary voltage relative to that of said test voltage.
 4. A method of testing an insulated winding incorporated in an electrical apparatus and pinpointing any fault which may exist in the insulation between any point along the winding and an electrically conductive component of the apparatus, the pattern of the voltage distribution along the winding being known, which comprises the steps of: applying a progressively increasing test voltage across the ends of said winding until a partial discharge strikes between the winding and the electrically conductive component of the apparatus, decreasing the test voltage until the partial discharge extinguishes, again increasing the test voltage to a level just above that where the partial discharge would first strike, subtracting from said test voltage an auxiliary voltage applied between one end of said winding and the electrical conductive component and which is varied until said partial discharge extinguishes again, and determining the location of the faulty turn in said winding where the partial struck from the ratio of the test the auxiliary voltages and the total number of turns in the winding.
 5. A method of testing an insulated winding as defined in claim 4 wherein said test and auxiliary voltages are of the alternating current type having the same frequency but displaced in phase by 180*.
 6. A method of testing an insulated winding as defined in claim 4 wherein said test and auxiliary voltages are of the alternating current type and have the same frequency and which includes the step of also varying the phase of said auxiliary voltage relative to that of said test voltage.
 7. A method of testing an insulated winding incorporated in an electrical apparatus and pinpointing any fault which may exist in the insulation between any point along the winding and an electrically conductive component of the apparatus, the pattern of the voltage distribution along the winding being known, which comprises the steps of: applying a progressively increasing test voltage between one end of said winding and the electrically conductive component until a partial discharge strikes between the winding and the electrically conductive component of the apparatus, decreasing the test voltage until the partial discharge extinguishes, again increasing the test voltage to a level just under that at which a partial discharge would again be struck, adding to said test voltage an auxiliary voltage applied across the ends of said winding and which is varied until said partial discharge re-strikes, and determining the location of the faulty turn in said winding where the partial discharge struck from the ratio of the test and auxiliary voltages and the total number of turns in the winding.
 8. A method of testing an insulated winding as defined in claim 7 wherein said test and auxiliary voltages are of the alternating current type and have the same phase and frequency.
 9. A method of testing an insulated winding as defined in claim 7 wherein said test and auxiliary voltages are of the alternating current type and have the same frequency and which includes the step of also varying the phase of said auxiliary voltage relative to that of said test voltage.
 10. A method of testing an insulated winding incorporated in an electrical apparatus and pinpointing any fault which may exist in the insulation between any point along the winding and an electrically conductive component of the apparatus, the pattern of the voltage distribution along the winding being known, which comprises the steps of: applying a progressively increasing test voltage between one end of said winding and the electrically conductive component until a partial discharge strikes between the winding and the electrically conductive component of the apparatus, decreasing the test voltage until the partial discharge extinguishes, again increasing the test voltage to a level just above that where the partial discharge would first strike, subtracting from said test voltage an auxiliary voltage applied across the ends of said winding and which is varied until said partial discharge extinguishes again, and determining the location of the faulty turn in said winding where the partial discharge struck from the ratio of the test and auxiliary voltages and the total number of turns in the winding.
 11. A method of testing an insulated winding as defined in claim 10 wherein said test and auxiliary voltages are of the alternating current type having the same frequency but displaced in phase by 180*.
 12. A method of testing an insulated winding as defined in claim 10 wherein said test and auxiliary voltages are of the alternating current type and have the same frequency and which includes the step of also varying the phase of said auxiliary voltage relative to that of said test voltage. 